In an EGR apparatus, an intake pipe and an exhaust pipe are linked through an EGR pipe so that exhaust gas is supplied through the EGR pipe to the intake pipe. Further, in the inside of the EGR pipe, an EGR valve is provided for controlling the flow rate of the EGR gas (the exhaust gas flowing through the intake pipe).
In a technique investigated presently, a differential pressure between the exhaust pressure and the intake pressure is detected and then the opening degree of the EGR valve is corrected in accordance with the differential pressure so that changing of the EGR rate caused by changing of the differential pressure is suppressed. An example of such a technique is, for example, a technique described in Patent Document 1 by the present applicant.
Here, the EGR rate, the EGR gas amount, the air intake amount, the differential pressure, and the opening degree satisfy the following relations shown in Formulas (1) and (2).EGR rate=EGR gas amount/(air intake amount+EGR gas amount)  (1)EGR Gas Amount=Proportionality Constant*Differential Pressure*Opening Degree Of an EGR Valve  (2)
The EGR rate is specified by detecting the air intake amount, the differential pressure, and the opening degree of the EGR valve. Therefore, when the opening degree is changed in accordance with changing of the differential pressure, the EGR rate can be maintained at a predetermined target rate.